JP4882256B2 - Thin film transistor - Google Patents

Description

  The present invention relates to a thin film transistor.

In recent years, thinned electronic devices such as organic EL, electronic paper, and RFID tags have been frequently used from the viewpoint of flexibility, weight reduction, and cost reduction.
These electronic devices are provided with an electronic circuit as in the prior art, and the electronic circuit is mainly formed of transistors.
In the manufacture of transistors using inorganic semiconductor materials such as silicon, many vacuum systems such as vacuum deposition, impurity doping, photolithography for pattern formation, etching, etc., also vacuum that strictly adjusts the degree of vacuum. The system is necessary, and the manufacturing cost of the transistor becomes very high due to the device cost and the running cost, and the price of the electronic device is also increased.

Therefore, in order to manufacture electronic paper, RFID tags, and the like at a low price, it is necessary to form a large number of transistors forming an electronic circuit on a flexible substrate at a low cost.
As this method, an organic transistor, in particular, an organic transistor formed using a printing method is used as a manufacturing method for forming on a flexible substrate at a low cost (see, for example, Non-Patent Document 1).
In other words, by using a printing method, transistors can be formed at low temperatures, so a resin film can be used as a flexible substrate, and a semiconductor is an organic substance, and a solution in which this is dissolved in a solvent is printed in the same way as printing ink. This eliminates the need for expensive equipment.
“The key is organic transistors. New materials exceed 1MHz”, Nikkei Electronics, February 16, 2004, p93-p113.

However, the above-described organic transistor has a problem that the transistor characteristics after the element formation becomes unstable because an organic semiconductor used for an active layer or the like is easily changed by light.
The present invention has been made in view of such circumstances, and an object of the present invention is to provide a thin film transistor using an organic semiconductor that has a small change in characteristics due to light and is excellent in light resistance.

  The invention according to claim 1 is a thin film transistor comprising a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the insulating film, wherein the gate insulating film is It is a reflective insulating layer having a light reflection characteristic, which is made of a material having a refractive index higher than that of the semiconductor layer.

  The invention according to claim 2 is a thin film transistor comprising a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the insulating film, wherein the gate insulating film is It is a reflective insulating layer having a light reflection characteristic, comprising a combination of a layer having a higher refractive index and a layer having a lower refractive index than the semiconductor layer.

  A third aspect of the present invention is the thin film transistor according to the second aspect, wherein the gate electrode and a layer of a material having a higher refractive index than the semiconductor layer are adjacent to each other.

  The invention according to claim 4 is the thin film transistor according to any one of claims 1 to 3, wherein the material of the layer having a higher refractive index than the semiconductor layer is a high refractive index fine particle having an average particle diameter of 2 to 50 nm. It is characterized by containing.

  A fifth aspect of the present invention is the thin film transistor according to any one of the first to fourth aspects, wherein the reflective insulating layer has an optical film thickness of 50 nm to 200 nm.

  The invention according to claim 6 is a thin film transistor comprising a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the insulating film, wherein the gate insulating film is It is characterized by comprising a gate insulating layer and a reflective layer having a light reflection characteristic, which is made of a material having a higher refractive index than that of the gate insulating layer.

  The invention according to claim 7 is a thin film transistor comprising a semiconductor layer, a gate insulating film formed on the semiconductor layer, and a gate electrode formed on the insulating film, wherein the gate insulating film is It is characterized by comprising a gate insulating layer and a reflective layer having a light reflection characteristic, which is a combination of a material layer having a higher refractive index and a material layer having a lower refractive index than the gate insulating layer.

  The invention described in claim 8 is the thin film transistor according to claim 7, characterized in that the gate electrode and a layer of a material having a higher refractive index than the gate insulating layer are adjacent to each other.

  A ninth aspect of the invention is the thin film transistor according to any one of the sixth to eighth aspects, wherein the material of the layer having a higher refractive index than the gate insulating layer is a high refractive index fine particle having an average particle diameter of 2 to 50 nm. It is characterized by containing.

  A tenth aspect of the present invention is the thin film transistor according to any one of the sixth to ninth aspects, wherein an optical film thickness of the reflective layer is 50 nm to 200 nm.

An eleventh aspect of the present invention is the thin film transistor according to any one of the first to tenth aspects, wherein the semiconductor layer is made of an organic semiconductor.
The invention according to claim 12 is the thin film transistor according to any one of claims 1 to 10, wherein the semiconductor layer is made of an oxide semiconductor.

  As described above, according to the present invention, the gate insulating film is a reflective insulating layer composed of a layer having a higher refractive index than the semiconductor layer, or a combination of a layer having a higher refractive index than the semiconductor layer and a lower layer. Adjacent to a layer having a higher refractive index than that of the semiconductor layer, an optical multilayer structure is formed, and incident light is reflected by the reflective insulating layer, thereby reducing the amount of light reaching the semiconductor layer and degrading the semiconductor layer. By suppressing the light resistance, the light resistance of the thin film transistor can be improved.

  Further, according to the present invention, the gate insulating film is composed of a gate insulating layer and a layer having a higher refractive index than the gate insulating layer, or a combination of a gate insulating layer, a layer having a higher refractive index than the gate insulating layer, and a layer having a lower refractive index. The light quantity reaching the semiconductor layer by forming an optical multilayer film structure by making the gate electrode adjacent to a layer having a higher refractive index than the gate insulating layer, and reflecting incident light by the reflective insulating layer The light resistance of the thin film transistor can be improved by reducing the degradation and suppressing the deterioration of the semiconductor layer.

In the thin film transistor of the present invention, the gate insulating film is formed of a material having a higher refractive index than the semiconductor layer, or the gate insulating film is formed of a material layer having a higher refractive index than that of the semiconductor layer and a layer of a material having a lower refractive index. In this case, a layer having a high refractive index is adjacent to the gate electrode (a gate insulating film is formed on the layer having a high refractive index), and an optical multilayer structure is formed to reflect incident light. A reflective insulating layer is formed.
In the thin film transistor of the present invention, the gate insulating film is formed of a gate insulating layer and a material having a higher refractive index than the gate insulating layer, or the gate insulating layer and a material having a higher refractive index than the gate insulating layer. Formed by combining a layer and a layer of a material having a low refractive index, and a layer having a higher refractive index than that of the gate insulating layer is adjacent to the gate electrode (the gate electrode is formed above the layer having a high refractive index). In other words, an optical multilayer structure is formed to form a reflective insulating layer that reflects incident light.

In the following, as an embodiment of the present invention, a case where a staggered TFT is used will be described. However, the present invention is not limited to this structure, and a thin film transistor having a layer structure of another gate electrode, a gate insulating layer, and a semiconductor layer. If so, it is effective in improving the light resistance of the semiconductor layer against light incident from the gate electrode toward the semiconductor layer.
The staggered thin film transistor shown in FIG. 1 has a structure in which a source / drain electrode 2, a semiconductor layer 3, a gate insulating film 4, and a gate electrode 5 are sequentially formed on a substrate 1.

In the gate insulating film 4, a layer having a higher refractive index than the semiconductor layer 3 (high refractive index layer) and a layer having a lower refractive index than the semiconductor layer 3 (low refractive index layer) have a high refractive index above the semiconductor layer 3. It is a laminated body formed in the order of the layer 41, the low refractive index layer 42, and the high refractive index layer 43, and forms a reflective insulating layer having an optical multilayer film structure.
Here, after the high refractive index layer 43 is formed, the gate electrode 5 is formed on the high refractive index layer 43 and is adjacent to the high refractive index layer 43.
Further, as shown in FIG. 2, the gate insulating film 4 is formed as a single layer using a material having a higher refractive index than that of the semiconductor layer 3, so that the gate insulating film 4 is adjacent to the gate electrode 5, thereby serving as a reflective insulating layer. It has a function.

Furthermore, as another configuration, as shown in FIG. 3, the gate insulating film 4 is formed of a gate insulating layer 44 and a high refractive index layer 45 made of a material having a higher refractive index than the gate insulating layer 44, and an optical multilayer film is formed. A reflective layer having a structure may be formed.
At this time, a gate insulating layer 44 is provided above the semiconductor layer 3, a high refractive index layer 45 is provided above the gate insulating layer 44, a gate electrode 5 is provided above the high refractive index layer 45, The high refractive index layer 45 is configured to be adjacent.

As another configuration, as shown in FIG. 4, the gate insulating film 4 includes a gate insulating layer 44, a high refractive index layer 46 made of a material having a higher refractive index than the gate insulating layer 44, and a gate insulating layer 44. A reflective layer having an optical multilayer structure may be formed by forming a low refractive index layer 47 made of a material having a low refractive index and a high refractive index layer 48 made of a material having a refractive index higher than that of the gate insulating layer 44.
At this time, the gate insulating layer 44 is provided above the semiconductor layer 3, the high refractive index layer 46 is provided above the gate insulating layer 44, and the low refractive index layer 47 is provided above the high refractive index layer 46. A high refractive index layer 48 is provided on the refractive index layer 47, and the gate electrode 5 is provided on the high refractive index layer 48, so that the gate electrode 5 and the high refractive index layer 48 are adjacent to each other.

In each of the thin film transistors described above, the base material 1 is not particularly limited, but various glass substrates or known plastic films or plastic sheets having a predetermined mechanical rigidity can be used such as heat resistance and flexibility. From the viewpoint, it can be appropriately selected and used.
Specifically, the glass substrate is soda lime glass, quartz or silicon wafer, and the plastic film or plastic sheet is polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polypropylene, cycloolefin polymer, polyimide, polyester. -Tellsulfone (PES), polymethyl methacrylate (PMMA), polycarbonate, polyallylate and the like can be used.

The source / drain electrodes 2 are formed, for example, by screen-printing conductive ink on the upper surface of the substrate 1 and then performing heat treatment as necessary, or by patterning a copper-clad substrate with a copper foil attached. be able to.
Further, the source / drain electrode 2 is formed on the upper surface of the substrate 1 by vapor deposition such as vacuum deposition, sputtering, CVD (chemical vapor deposition), letterpress printing, intaglio printing (gravure) printing, planographic printing, ink jet printing, etc. It may be formed by other printing methods.
Here, as the conductive material of the source / drain electrode 2, metal materials such as Ni, Al, Cu, Ag, Au, or fine powders such as Ni, Al, Cu, Ag, Au, and carbon, and Ag, Cu, Various conductive pastes containing conductive materials such as nanoparticles such as Au and organic Ag compounds, and known materials such as conductive ink can be used.

The semiconductor layer 3 can be formed of various known semiconductor materials, and examples of the organic material include bentacene, polythiophene, polyallylamine, and fluorenebiothiophene copolymer. According to the present invention, photodegradation of organic semiconductors is particularly effectively suppressed, but inorganic semiconductors are effective to some extent. Examples of the inorganic semiconductor include carbon compounds such as carbon nanotubes and fullerenes, oxide materials such as InGaZnO-based, InGaO-based, ZnGaO-based, InZnO-based, ZnO, and SnO2, and metal chalcogenide compounds such as cadmium selenide.
The semiconductor layer 3 can be formed by various known methods, for example, spin coating, dip coating, screen printing, letterpress printing, intaglio printing, planographic printing, ink jet, sputtering or laser ablation, metal organic chemical vapor deposition. An appropriate vacuum evaporation or the like is appropriately selected according to the material.

Hereinafter, description will be made with reference to the structure of FIG.
The gate insulating film 4 is a three-layered structure including a high refractive index layer 41, a low refractive index layer 42, and a high refractive index layer 43.
Further, the gate insulating film 4 is not limited to the number of layers of the low refractive index layer and the high refractive index layer shown in FIG. 1, and includes at least a high refractive index layer as a stacked body (high refractive index layer). Includes a single layer of only one layer), a high refractive index layer 41 (43) and a low refractive index layer 42 are alternately stacked.
The optical multilayer film structure of the gate insulating film 4 causes interference of incident light from the outside, and reflects it to reduce the amount of incident light transmitted from the gate electrode 5 side when reaching the semiconductor layer 3. It is possible to suppress deterioration of the characteristics of the semiconductor layer 3 due to incident light from the outside.

The high refractive index layer and the low refractive index layer used for forming the gate insulating film 4 can be formed by a process corresponding to each material using a known material.
Here, for the high refractive index layers 41 and 43, for example, titanium dioxide, zirconium dioxide, cerium dioxide, indium oxide, tantalum oxide, zinc oxide, or the like can be used.
For the low refractive index layer 42, for example, silicon dioxide, magnesium fluoride, calcium fluoride, or the like is used.
These film forming methods are not particularly limited, and wet processes such as spin coating and die coating, or dry processes such as sputtering, vacuum deposition, and CVD can be used.

In the wet process described above, the coating liquid for forming the low refractive index layer 42 is not particularly limited, but various soluble fluororesins, silicon alkoxides and hydrolysates thereof can be used.
Moreover, in order to lower the refractive index of the low refractive index layer 42 to the coating liquid, low refractive index particles such as silicon oxide and magnesium fluoride may be added.
On the other hand, the coating liquid for forming the high refractive index layers 41 and 43 preferably contains high refractive index particles (high refractive index fine particles) having an average particle diameter of 2 to 50 nm in order to sufficiently increase the refractive index.

Here, by increasing the refractive indexes of the high refractive index layers 41 and 43, the amount of incident light transmitted from the outside reaching the semiconductor layer 3 can be effectively reduced.
Examples of the high refractive index particles contained in the coating solution for forming the high refractive index layer include titanium dioxide, zirconium dioxide, cerium dioxide, indium oxide, and tantalum oxide.
The high refractive index particles having an average particle diameter of 2 nm or less have low crystallinity and the refractive index is not so high as to improve the refractive index of the entire layer, and the refractive index of the formed high refractive index layer is sufficiently high. Can not do it.
On the other hand, high refractive index particles having an average particle diameter of 50 nm or more are problematic in that the insulating properties are deteriorated and the adverse effect on the performance of the thin film transistor due to surface irregularities.

  The various coating liquids contain a binder component as necessary, and as this binder, organic materials such as polyester / melamine resin paste, polymethyl methacrylate, polyvinyl chloride, polyvinyl alcohol, polyvinyl phenol, polystyrene, cyanoethyl pullulan, Various metal alkoxides such as silicon, aluminum, and titanium, and inorganic materials composed of hydrolysates thereof can be appropriately selected and used.

The optical film thicknesses of the high refractive index layers 41 and 43 and the low refractive index layer 42 are preferably 50 to 200 nm, respectively.
By setting the film thickness within this range, a minimum light transmittance appears at approximately 200 to 800 nm in the light transmission spectrum.
That is, it is possible to particularly reduce the light transmittance in the ultraviolet-visible region that causes the light degradation of the semiconductor layer 3.

In particular, when the optical film thicknesses of the high refractive index layers 41 and 43 and the low refractive index layer 42 are set to 50 to 200 nm, respectively, when an organic semiconductor material having low light resistance is used as the semiconductor layer 3, suppression of light degradation is performed. Great effect.
Here, when the optical film thickness is 50 nm or less, the wavelength at which the minimum appears in the light transmission spectrum becomes too short, and the transmittance in the ultraviolet light region cannot be effectively reduced.
On the other hand, when the optical film thickness is 200 nm or more, the transmittance in the ultraviolet-visible region may be reduced in a predetermined film thickness range, but the gate insulating film 4 becomes too thick as a multilayer film and is applied to the gate electrode 5. This is not preferable because the driving voltage becomes high.

Further, the number of layers of the gate insulating film 4 is not limited, but there is a merit that the decrease in transmittance due to optical interference increases as the number increases. Since 4 becomes thicker and the drive voltage applied to the gate electrode 5 becomes higher, care must be taken in designing the film thickness and the number of layers of each refractive index layer corresponding to the drive voltage.
Similarly to the source / drain electrode 2, the gate electrode 5 is formed by patterning various electrode materials on the high refractive index layer by various dry processes and various printing methods.

The present invention is also applicable to a planar type thin film transistor.
As shown in FIG. 5, the planar type thin film transistor of the present invention is provided with a pattern of a gate electrode 5 on a base material 1, a gate insulating film 4 is provided on the top, a semiconductor layer 3 is provided, and a semiconductor layer 3 is provided on the top. Although the source / drain electrodes 2 are sequentially formed, when the gate insulating film 4 is configured as described above, when the incident light from the gate electrode 5 side reaches the semiconductor layer 3, this light transmission amount Can be reduced.

<Example of Fabrication of Thin Film Transistor According to an Embodiment of the Present Invention>
The staggered thin film transistor shown in FIG. 1 was produced by the following procedures (1) to (5).
(1) Formation of Source / Drain Electrode 2 On the glass substrate which is the base material 1, Ag is mask-deposited into the pattern shape of the source / drain electrode 2, and the source / drain electrode 2 is formed to a thickness of, for example, 50 nm. , And having an interval of 20 μm.
(2) Formation of Semiconductor Layer 3 A poly (3-hexylthiophene) solution is spin-coated on the base material 1 on which the pattern of the source / drain electrodes 2 is formed, and then the poly solution is dried, thereby producing a semiconductor. Layer 3 was formed to a thickness of 0.5 μm. Here, the refractive index of the semiconductor layer 3 was calculated to be about 1.6 as a result of estimation from the reflectance.

(3) Formation of gate insulating layer 4 a. Preparation of coating solution for forming high refractive index layer (41, 43) Isopropanol-dispersed titanium oxide particles (high refractive index component), titanium tetrabutoxide (binder component), toluenesulfonic acid (hydrolysis catalyst), isopropanol (solvent) ) Were mixed to prepare a coating solution as a highly refractive coating solution.
In the high refractive coating solution, the molar ratio of titanium oxide particles to titanium in the binder was titanium oxide / binder = 8/2, and the solid content concentration was 3 wt% in terms of titanium oxide. The refractive index of the high refractive index layers 41 and 43 formed from the high refractive coating liquid was determined to be about 1.95 as a result of estimation from the reflectance.

b. Preparation of coating solution for forming low refractive index layer (42) Tetramethoxysilane / tridecafluorotrimethoxysilane / 0.1N hydrochloric acid was mixed at a molar ratio of 9/1/30, hydrolyzed, and then isopropanol. It diluted and prepared the coating liquid as a low refractive coating liquid.
The solid content concentration of the low refractive coating solution was 3 wt% in terms of silicon dioxide.
The refractive index of the low refractive index layer 42 formed from the low refractive coating liquid was determined to be about 1.35 as a result of estimation from the reflectance.

c. Formation of Gate Insulating Film 4 On the semiconductor layer 3, a high refractive index layer 41 is coated and dried as a high refractive index layer 41, and then a low refractive index layer 42 is coated and dried as a high refractive index layer. The high-refractive coating liquid was applied and dried again as 43, and the gate insulating film 4 was formed as a three-layer (multilayer) film of the high-refractive index layer 41, the low-refractive index layer 42, and the high-refractive index layer 43.
Here, the above application was performed by spin coating, and the drying condition was left at 150 ° C. for 10 minutes for each layer.
Further, the optical film thicknesses of the high refractive index layers 41 and 43 and the low refractive index layer 42 were each formed to be about 80 nm.

d. Formation of the gate electrode 5 As a material of the gate electrode 5, a conductive paste containing Ag particles is screen-printed on the gate insulating film 4 in a pattern shape of the gate electrode 5 at a thickness of 10 μm and then heat-treated to volatilize the solvent. Thus, the gate electrode 5 was formed.
Next, each electrode (source / drain electrode 2 and gate electrode 5) was provided with a bonding pad of a predetermined metal to form a thin film transistor element.

<Comparison of light resistance in transistor configuration between this embodiment and conventional one>
A top gate type thin film transistor is manufactured in the same procedure as the embodiment of the present invention except that the gate insulating film is a thin film having a refractive index of 1.6 and an optical film thickness of 240 nm as a conventional thin film transistor configuration. did.
The coating liquid for forming the gate insulating layer used in the conventional structure has a composition in which titanium oxide / binder = 0/10 in the formulation of the high refractive coating liquid.

As an evaluation result method, in the element configuration produced in this embodiment and the conventional example, each electrode (source / drain electrode 2 and gate electrode 5) and bonding pad are not provided, that is, base material 1 / semiconductor layer 3 / gate. An element composed of the insulating layer 4 was produced, and the light transmittance was measured using a spectrophotometer. In the transmittance measurement, light was incident from the gate insulating film 4 side.
For the incidence of this light, a handy ultraviolet lamp (wavelength: 365 nm, intensity: 650 μW / cm 2) was used. Immediately after the production of the thin film transistor element, ultraviolet light was irradiated for 2 hours from the gate electrode 5 side of the thin film transistor.

Then, the ON / OFF ratio is measured and evaluated based on the gate voltage dependence of the IV correlation of the device irradiated with ultraviolet rays, and compared with the case where it is stored in a dark place for 2 hours immediately after the device is manufactured. Is shown as an evaluation table in FIG.
Referring to the evaluation table, when the gate insulating layer 4 is an optical multilayer film, the change in the on / off ratio due to ultraviolet irradiation is reduced, and the light resistance is improved.
From this result, it can be seen that the light transmittance at a wavelength in the ultraviolet region is reduced, and the light deterioration of the semiconductor layer 3 is suppressed.

It is a conceptual diagram which shows the cross section of the thin-film transistor of the structure by one Embodiment of this invention. It is a conceptual diagram which shows the cross section of the thin-film transistor of the other structure in one Embodiment of this invention. It is a conceptual diagram which shows the cross section of the thin-film transistor of the other structure in one Embodiment of this invention. It is a conceptual diagram which shows the cross section of the thin-film transistor of the other structure in one Embodiment of this invention. It is a conceptual diagram which shows the cross section of the thin-film transistor of the other structure in one Embodiment of this invention. 4 is a table showing a comparison result of light resistance between a thin film transistor having a structure according to an embodiment of the present invention and a thin film transistor having a structure according to the prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Base material 2 ... Gate-source electrode 3 ... Semiconductor layer 4 ... Gate insulating film 5 ... Gate electrodes 41, 43, 45, 46, 48 ... High refractive index layer 42, 47 ... Low refractive index layer 44 ... Gate insulating layer

Claims (6)

A semiconductor layer;
A gate insulating film formed on the semiconductor layer;
A gate electrode formed on the insulating film;
Wherein the gate insulating film is the first gate insulating layer, formed on top of the first gate insulating layer, a combination of a first high refractive index insulating layer made of a material having higher refractive index than said first gate insulating layer , Ri reflective insulating layer Tona of the first reflective layer having a light reflection characteristic,
The first high refractive index insulating layer is formed of a material having a higher refractive index than the semiconductor layer, and the first gate insulating layer is formed of a material having a lower refractive index than the semiconductor layer, and the first high refractive index insulating layer is formed. A thin film transistor , wherein the gate electrode is formed on a layer . A second high refractive index insulating layer made of a material having a higher refractive index than the first gate insulating layer and a second material made of a material having a lower refractive index than the second high refractive index insulating layer are provided below the first reflective layer. A second reflective layer comprising a gate insulating layer is provided; the first gate insulating layer and the second high refractive index insulating layer are in contact; and the second high refractive index insulating layer has a refractive index higher than that of the semiconductor layer. The second gate insulating layer is formed of a material having a refractive index lower than that of the semiconductor layer, and the reflective insulating layer is formed together with the first reflective layer. A thin film transistor according to 1. It said height have materials of refractive index than the semiconductor layer, a thin film transistor according to claim 1 or claim 2, characterized by containing a high refractive index fine particles having an average particle diameter of 2 to 50 nm. 4. The thin film transistor according to claim 1, wherein the reflective insulating layer has an optical film thickness of 50 nm to 200 nm. 5. The thin film transistor according to any one of claims 1 to 4, wherein the semiconductor layer is characterized by comprising the organic semiconductor. The thin film transistor according to any one of claims 1 to 4, wherein the semiconductor layer is made of an oxide semiconductor.

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